Core D constitutes a centralized resource for packaging retroviral expression constructs into viral particles of choice (gamma-, lenti-, and foamy viruses). Services provided by this resource include: a) scalable viral production services with viral preps ranging from 2 x 10 to 2 x 10 transducing units, b) titer assessments by either fluorescent marker tracking or by real time PCR analysis, c) consultation for investigators regarding the appropriate design of the retroviral expression construct, and d) provision of control expression constructs. Core D in collaboration with Cores C, E, and F will also investigate new strategies for marking and detecting progeny of individual stem cells.